1. Field of the Invention
The present invention relates to a sheet feeder that feeds sheets one by one from a sheet bundle stacked in a sheet storage section.
2. Description of the Related Art
To date, in image forming apparatuses such as copying machines and printers, cut sheets serving as recording media that can be continuously fed have usually been limited to high quality paper or to ordinary paper designated by copying machine makers. Because such paper is low in surface smoothness and air-permeability, air easily enters between sheets, so that, when paper is taken out from stacked paper one by one, a double-feed due to paper-to-paper adhesion rarely occurs.
In recent years, however, with the diversification of recording media, there has been growing requests to form an image on cardboard, OHP paper, and tracing paper, and in addition, on art paper, coated paper, and the like, each of which has a smooth surface and each of which has been subjected to a coating treatment on the sheet surface thereof in order to produce gloss and to increase the degree of whiteness based on the market demand for colorization. However, OHP paper, tracing paper, art paper, coated paper, and the like each have high smoothness and air-permeability, and therefore, especially when sheets are stacked in a high moisture environment, double-feed and/or misfeed frequently occurs due to sheet-to-sheet adhesion when using the friction separation method, which has been generally used for conventional copying machines, printers, and the like.
On the other hand, the air sheet feeding method in which sheets are handled by blowing air from a side edge of a paper bundle, and in which the sheets are fed one by one from the sheet located in the uppermost position by a suction unit and a conveying belt that are each disposed above the sheet bundle, has been adopted in printing industry, or used for some copiers. Over the friction separation method, this air sheet feeding method has advantages as follows: 1) a wide setting latitude of paper feed conditions for materials, 2) adaptability for high-speed feeding, 3) high durability, and 4) lower running cost.
Such proposals related to the air sheet feeding have been made in large numbers. For example, Japanese Patent Laid-Open No. 62-249835 discloses a method in which air is blown by an air blowing unit from the direction parallel with the top surface of a sheet toward the front end side of sheets stacked on a paper feed stand, and that the pressure within a suction cylinder opened above sheets is made negative by a negative pressure generating unit to create suction pulling the sheet upward. In this paper feeding device, the sheet located in the uppermost position of the sheets stacked on the paper feed stand is lifted up by a suction action from the opening, and air is blown into a space formed between the sheet located in the uppermost position and that located in the second position by the air blowing unit, in order to reliably perform the separation between the two sheets.
However, as described above, since OHP paper, tracing paper, art paper, coated paper, and the like each have high smoothness and air-permeability, sheet-to-sheet adhesion is high especially when sheets are stacked in a high moisture environment. Here, when the air sheet feeding method is adopted, the uppermost portion of a sheet bundle is lifted off in the state of a bundle at an interface having a relatively weak adhesive force because air blown from the side edge of the sheet bundle is a steady flow. As a result, it has been impossible to inject air into a space between sheets of the lifted sheet bundle, thereby making it very difficult to reliably separate sheets one by one.